High alpha cellulose fiber and process of producing same



' coloring'substance conta Patented .Apr; a

UNITED; STA

enonen aiucn'rna, or 31mm, NEW

nmrsmrgn,

ASSIGNOB- 'ro BROWN comrany,

OF BERLIN, NEW"H.AMPSHIBE, A CORPORATION OF MAINE HIGH ALPHA CELL'U'LOSE Fl iBER ANDPBOCESS OF PRODUCIN G SAME No Drawing.

This invention has relation to high alpha cellulose fiber (i. e., pulp containing a high percentage of alpha or resistant cellulose) and .a process of producing the same more espe 'cially from chemical wood pulps, such as sulphite pulp, in an economical and efiicient manner. Before proceeding to a description of the present invention, inorder to give a clearer and more sufiicient understanding thereof,

and the results achieved thereby, it is desirable to consider the subject matter of a, plication for U; S. Letters Patent Serial No. 2,522, filed December 1, 1925, by Milton O. Schur 1 and myself.

In that application, it is pointed out that high alpha cellulose fiber may be produced by the purification of sulphite fiber, such as results from the digestion of wood chips in an acid sulphite liquor [as, for example, calcium bisulphite, Ca(HSO and which contains ordinarily about 85% to 87% alpha cellulose. That is,,since the entirely effective in removing the lignocellulo'ses contained in the wood, the resulting sulphite pulp still contains substantial amounts of the less-resistant beta and gamma celluloses, together'with gums, hgneous and *othercoloring substances, which are undes1ra ble constituents, if sulphite pulp is to be an ployed for certain purposes. In order to produce pulp which consists substantially only of alpha cellulose, .it is necessary to process sulphite pulp further, is practicable, the less-resistant celluloses,'together with coloring substance contained therein. The removal of some of the undesirable constituents of sulphite pulp'may be accomplished by the use of an excessive amount of bleach in the bleaching process, but this ishlghly detrimental, in that the alpha cellulose content of the pulp thereby suffers a material decrease by degradation into oxycellulose. Moreover, excessive bleach hasthe tendency to weaken the strength ofthe pulp. I" have discovered that an alkaline liquor is efiective in reacting g with and dissolving the less-resistant celluloses, including ligno-cellulose and: other bisulphite digestion is not to eliminate, so far as the residual ligneous and other ed iasulphite pulp Application filed August 24, 1925. Serial F0. 52,232.

and in the described a process ofproducing high alpha cellulose fiber by an alkaline digestion of unbleached sulphite pulp, preferably in a caustic soda or soda ash solution. The alkaline digestion, in such case, is carried out in a dilute solutionof caustic soda at an elevated temperature until the desired purification of the pulp has been effected. The purified pulp, containing, say, about 94% alpha cellulose, is washed free fromv the spent ,dlgestlng liquor, and has a high whitecolor after moderate bleaching. In carrying out the hot purification process of that application, a sulphite stock suspension at a consistency of about 9% to 12%, containing about 5% to 8% caustic soda, based on the weight of bone-dry fiber, is digested in open tanks under atmospheric pressure at a temperature of about 180 to 212F., for about three to eight hours, while preferably agitating the stock and maintaininglthe elevated temperature by blowing live steam into the tanks.

' The primary object of the present invention is toprodiuce a high alpha'cellulosefiber of substantially equal or betterquality than that obtained, by a hot alkaline digestion of unbleached sulphite pulp and in a more economical manner. I have discovered that this object may be attained by treating chemical wood pulp, such as sulphite pulp, with a con.- centrated solutiofi of caustic soda, at room temperature (say, about 30 0.), the solution containing sufiicient alkali to react with the less-resistant celluloses, ligneous, and other coloring substance contained in the pulp, but

insuflicient to cause a substantial mercerlza aforementioned application have tion of the alpha cellulose-content thereof;

' In carrying out the gold purification of sulphit'e pulp, too high a concentration of caustic soda must be avoided,'since otherwise the fibers will become mercerized, andin so doing will swell and acquire a coating of mercerized cellulose. 'Mercerizationlowers the strength of the pulp; and' on beating, mercerized pulp has no tendency to hydrate or elatinize, beating merely macerating or cutting the-fiber, so that paper made fromthe V beateno'mercerized fiber is extremely weak.

Onthe other hand, if toolowa caustic conupon the exact temperature employed and the characteristics of the raw stock.

After the purification, the pulp, containing preferably at least 9t% alpha cellulose, is washed free from alkaline agent, and may be bleached to a high degree of whiteness with a moderate amount of hypochlorite bleach,

without material inj to its strength or de-' crease in its ,alpha cellulose content. If a pulp of maximum whiteness is desired, the bleached pulp may be superbleached in a chlorine solution.

I have found that the time required for such cold purification, using strong solutions of caustic soda, is materially shorter than a corresponding hot treatment, so that with a given apparatus, a much higher production may befrealized. Moreover, the necessity for the use of'steam is avoided, thus eliminating this factor from entering into the production cost of the finished product. A further important advantage is that for a given yield of pulp, the alpha cellulose content is higher,

and the pentosan content lower.

While I have indicated that the process is ordinarily and preferably carried out at room temperature, 1 desire to have it clearly understood that lower than room temperature,

say 0 C. to C and a correspondingly suitable caustic soda concentration, may be employed to effect the purification of the sulphite pulp. Likewise, higher temperature (for example, C. to C.) and a correspondingly suitable concentration of caustic soda may be employed in such treat- -ment. I have further discovered that a concentrated solution of sodium sulphide when used in lieu of a caustic soda solution will also effect a purification of sulphite pulp at relatively low temperature. However, in securing a purification" equivalent to that obtainedby a caustic soda solution, the sodium sulphide solutionmust contain about twice as' much purifying sodium sulphide (by weight) as the caustic soda solution does caustic soda; and this renders the sodium sulphide treatment more expensive than the caustic soda treatment.

At any temperature, the maximum concentration of caustic soda which may be employed must be below that at which mercerization occurs, and the minimum above that at which a sufiicient purification is not cffected. For optimum results in respect of alpha cellulose contentin a finished product produced from unbleached sulphite pulp, ata temperature of about 30 (3., the minimum and maximum concentrations in a stock suspension of about 5% to 6% are, respectively, about 100% and-150% caustic soda, based on the weight of dry fiber. The alkalinity of the caustic soda solution is thus about 5% to 7 caustic soda strength. i

The cold alkaline treatment may be practiced with advantage after a portion of the ligneous and other coloring substance contained in the sulphite pulp has been removed, by treating such pulp with an oxidizing solution, preferably a solution containing chlorine. Such pretreatment is especially desirable when the sulphite pulp is undercooked, since such undercooking leaves a higher percentage of ligneous and other coloring substance than ordinarily. The oxidizing agentreacts with a portion of the impurities, forming soluble reaction products or products which are easily removable byv the alkaline treatment. In carrying out the process, wood chips (spruce, for example) are cooked in a digester in an acid sulphite liquor [as, for example', Gag-I80 or N aHSO of a strength of about-1% combined and.5%6% free S0 under the requisitetime, temperature, and 1 pressure conditions in accordance with standard modern practice. The acid digestion is especially effective in removing ligno-celluloses and coloring substance from the Wood, but leaves behind a certain residual percentage thereof, together with less-resistant cellulose and some pentosan, the resulting sulphite'pulp containing about 85% to 87% al pha cellulose. After digestion, the digester is blown into a suitable blowpit; and the sulphite pulp is separated from its digesting liquor, which may be treated, if desired, for the recover of chemicals. The pulp is then w shed su stantially free of its digesting. liq or and the spent products of digestion, and is preferably screened to remove specks, shives. etc., prior to the alkaline digestion. It may also be partially bleached, as hereinafter described, prior to the alkaline digestion. It is then thickened by any suitable form of pulp thickener, and intimately admixed with analkaline solution, preferably caustic soda, to a consistency of about 5% to 6%, the suspension containingpreferably about to 150% caustic soda, based on the weight of dry fiber. While a lower percentage of caustic soda may be employed in cases where the resulting pulp need not contain at least 94% alpha cellulose, in the practice of the present invention, it is preferable to employ a caustic soda concentration between the limits indicated, so that a suflicient purification of the fiber may be obtained without impairing or injuring its paper-making characteristics to a sulting product, which .may contain fronr 94L-%*-to 98% alpha cellulose, is then washed substantially free from alkali. The spent liqhor contains not ,only a very high perce itage of residual unconsumed caustic soda, but alsothe various organic substances removed from the sulphite pulp as indicated. The large portion of spent liquor is reemployed for the treatment of other sulphite pulp.

The washed pulp is -of light color, not much difierent in appearance from the color of or-' dinary unbleached sulphite ulp.

As indicated, it may be esirable to treat the sulphite pulp with a solution of an oxidizing agent prior to the alkaline treatment, especially when such pulp is of arefractory or undercooked nature and thus has a higher ligneous content than ordinarily. In such cases, the pulp is treated, approximately at room temperatu e at a stock density'of 5% subsequent alkaline treatment. The pre- .ous acid solution, to neutralize and thereby (ill.

for about an ifiur, preferably in a solution containing 1% to 2% chlorine, or 2% to 6% of 35%lime bleach (CaOCl based onthe dry weight offiber. The solution reacts with the ligneous and coloring substance contained in the pulp, forming soluble reaction products easily or products removable by the treated stock is then treated in a concentrated solution of caustic. soda, as hereinbefore described. i

Subsequent to, or during the washing of the alkaline treated stock, it may be desirable to add a sniall amount of bisulphite or sulphurremove any last traces of alkali which the washing does not eliminate,the additionof 'weakacid being especially desirable if the washing is incomplete, since in such a case there is apt to be an appreciable quantity of caustic soda present, which would otherwise cause an alkalinecondition in the bleaching. If alkali is necessary in the bleaching operation, it is better to provide alkali for the pur-- pose, as willsubsequently appear,rather than to'depend on the alkali present in an incompletely washed pulp. In actual practice, the isulphite or sulphurous acid may or maynot be used prior to bleaching, according to economic expediency.

Since the alkaline treatment has removed from the original fiber a substantial amount of ligneous and other coloring matter, to

getherwith less-resistant beta and gamma celluloses, the bleaching of; the fiber may be accomplished with less hypochlorite bleach of a white product. The stock is treated at a consistency of about 10% to 16% with about 5% to 10% of 35% lime bleach (09.001 based on the dr weightof pulp. Precautions must be ta en to maintain a tempera-' turenot exceeding 90 F. in the bleaching tanks, in order to preserve the maximum strength of fiber. Ii desired, a certain small pro ortion of ustic soda liquor may be added o'the bleac liquor and ulp within the bleachery, to retard the bleac ing and there; by avoid the possibility of lowering the alpha cellulose content or injuring the strength of the stock due to too-violent bleaching. The bleaching period ranges from eight to twenty hours, dependin upon the characteristics of the fiber undergoing treatment; and at its conclusion the stock'has a color of 95- to 100. The bleached high alpha cellulose fiber has approximately the following characteristics:

Yield (from unbleached sulphite) .a 89-92% Alpha cellulose 94-98% Average fiber length 1. -i. 2 min.

The stock thus bleached may be improved by a superbleaching operation. In superbleaching, bleached high alpha cellulose fiber is treated at a stock consistency of from 4% to 6%, with about 0.2% to 0.5% chlorine, based on the weight of bone-dry fiber. No efiort need be made to control the temperature in superblea'ching, which may approximate the temperature of water available at the time. If it is desired to increase the rate of superbleaching, the to not higher than 90 F. After superbleaching, the stock is treated with an antichlor, as for example, a bisul hite solution, which serves to react with an to neutralize. any residual traces of chlorine or hydrothe preferably washed and temperature may be increased chloric acid present in the stack. The stock is then washed substantially free from reactipn products. The supegbleaching may be carried out on stock which has not been washed after its main bleaching operation, but this is not altogether a satisfactory mode of operation, due to the high chlorine consumption resulting from the reaction of the chlorine with the products of reaction of the original bleach. The superbleaching requires i to 2 hours, and yields a stock having a color 019106 to 109.

- The bleached or superbleached stock is then diluted to about 2% to 4%, and is deliver'ed 'over a set of rifiers, which removes from the stock a substantialper cent of heavy impurities, such as cinders, cement, or iron rust, which maybe present therein in small uantities. It may then be sent to a classi- .treatment which it has received as described hereinbefore, high alpha cellulose produced by the practice of my inventiongnay be employed instead of rag fiber in the manufacture of high grade bond and ledger papers, as asubstitute for cotton fiber in the manufac ture ofthe cellulose esters and, the cellulose ethers, and for any other purpose where a substantially pure cellulose fiber is required. The relatively small per cent of short fiber high alpha cellulose obtained in classification may be used to advantage in the preparation of cellulose derivatives.

Another advantage following from the practice of'the herein described process lies in the fact that, with the high alpha cellulose fiber obtained therefrom, a solution of cellulose or cellulose esters may be obtained of lower viscosity than it is possible to produce either with ordinary cotton fiber or with high alpha cellulose produced. by the hot process. That is, with a given amount of cellulose in solution, a solution of high alpha cellulose produced by the cold method is of a? materially lower viscosity'than a solution of cotton, and of considerably lower viscosity than a solution of high alpha cellulose pro-" duced by the hot process. Consequently, when solutions of cellulose or cellulose esters are employed in operations where a high concentration of cellulose together with a low viscosity'is desirable, and when the concentration to be employed is limited by the viscosity of the solution, the high alpha cellu-' lose fiber as herein produced may be employed at a much-higher concentration and with moreadvantageous result than cotton or high alpha cellulosev produced by the hot process.

As previously indicated, whilehigh alpha cellulose fiber may be produced by a cold caustic soda solution treatment of sulphite pulp formed either by the acid digestion of wood chips in a sodium bisulphite liquor orin a calcium bisulphite liquor, I preferably employ sulphite pulp resulting from the sodium base liquor,'because of the more desirable physical characteristics in the final, product. .In this connection, it may be stated that while the high'alpha cellulose fiber produced from the sulphite pulp liberated from wood chips by a sodium base acid liquor possesses'an alphacellulose content substantially the same. as'that produced from acalcium base acid liquor, its physical characteristics,

' ;which has been produced including strength, tear and folding endurance, are materially higher. This is undoubted ly due to the fact that the unbleached sul- 'phite pulp resulting from a sodium base sulphite liquor is superior in quality to a pulp obtained from a calcium base sulphite liquor because of its greater fiber length. The greater length of fiber and better strength are believed in great measure to be due to the more thorough penetration of a sodium base sulphite liquor and the lower maximum temperature of digestion required forconversion of the wood chips to pulp. The superior physical characteristics of the sodium base fiber carry through the various treatments, including the cold alkaline treatment, to the finished high alpha cellulose stock. In fact, its tear test is increased relatively by the cold alkaline treatment to a much greater extent than the increase in the tear test of a pulp produced in an acid calcium bisulphite liquor.

lVhile I have confined myself to a description of the purification of unbleached sulphite pulp, I have also discovered that the characteristics of pulps of various other derivations may be favorably modified-by treatment with a concentrated caustic soda solution in the cold. For example, as disclosed in my application, Serial No. 73,193, filed December 4, 1925, kraft or sulphate pulp may be similarly treated or as disclosed in my application Serial No. 2,602 filed Decemberl, 1925, a 4-4 pulp, i. e., a pulp by digestion in an acid sulphite liquor containing a proxi mately 4% combined and 4% free S may be similarly treated. In each case a product possessing certain desirable characteristics is produced. I do not herein claim broadly a process wherein pulp is treated with an alkacaustic; soda,as specified in application, Serial No. 72,522, filed by Milton 0. Schur and myself on December 1, 1925; nor do I claim herein the process or. parts thereof generically or specifically claimed therein.

What I claim is:

I 1.; A process which comprises treating sul- 1 phite pulp admixed with a solution of caustic soda to a stock consistency of about 5% to 6% at about room temperature for about thirty minutes to two hours, said solution line reagent,-specifically a hot solution of- Ill containing to caustic soda based on thR weight of dry fiber.

2. V phite pulp at about room temperature with ing caustic soda in amount sufiicient to react with and render soluble less-resistant celluloses a-nd lign'eous matter contained therein,

process which comprises treating sula concentrated caustic soda solution contain,

but 'ins'uflicient to cause substantialmereerisaid washed pulp at about room temperaturp with a concentrated caustic soda solution containing caustic soda in amount sufficient ill ' pulp.

to react with and render soluble less resistant celluloses and ligneous matter contained therein, but insufiicient tocause a substantial mercerization of the alpha cellulose content thereof. 1 v 4. A process which comprises treating sulphit-e pulp at approximately room temperature at a stock density of about 5% in a solution containing 1% to 2% chlorine based on the dry weight of fiber, washing said pulp free from said solution, and treating said washed pulp at a-stock density of 5% to at about room temperature in a solution containing about 100% to 150% caustic soda based on the dry weight of fiber. 5. A process which comprises treating sulphite pulp with a caustic soda liquor containing from 5% to 8% caustic soda at about room temperature for about thirty minutes to two hours, thereby efiecting a solution.

of a substantial proportion of the non-alpha cellulose content of such pulp 6. A'process which comprises treating sulphite pulp at approximately room temperature at a stock density of about 5% in a".

solution-containing 1% to 2% chlorine based on the dry weight of fiberpwashing said pulp free from said solution and treating said washed pulp with'a causticsoda liquor containing from 5% to 8% caustic soda'at about room temperature.

7. A process which cellulose pulp liberated from raw cellulosic material at a tem rature considerably below alkalinity to ehe'ct a solution of a substantial-proportion of the non-alpha cellulose content of such pulp without deleteriously affecting its papermaking characteristics.

8., A process which comprises treating pulp liberated from raw cellulosic material at about room tem' rature with a caustic soda,

solution of suficient alkalinity to react upon and render solublea substantial proportion ofthe non-alpha cellulose content ofthe pulp, but of insufiicient alkalinity tdmercerize the 9: A. process which qomprises treating chemical wood pulp at about room temperature with a caustic soda solution of sufiicient alkalinity toreact upon and render soluble a substantial proportion of the non-alpha cellulose' content of the pulp, but ofinsufiicient alkalinity to mercerize the pulp, washing the comprises treating pulp substantially free of-\the solution and dissolved reaction products, and bleaching the (washed product.

10. A process which'comprisestreating pulp liberated from raw cellulosic material,

at* about room temperature with an alkaline solution of sufficient alkalinity to effect a solution ofa substantial pro ortion of the non-alpha cellulose content 0 such pulp butv without deleteriously afl'ecting its papermaking characteristics.

' 11. A. process which comprises treating chemical wood pulp at about room temperature with an alkaline solution containing caustic soda and sodium sulphide in amount sufficient' to efiect a solution of a substantial proportion of the non-alpha cellulose content of such pulp but without \deleteriously at fecting its papermaking characteristics.

' 12. In purifying unbleached chemical wood pulp, the sequence of operations which com prises partially bleaching the pulp, treating the partially bleached pulp with a solution of caustic alkali, the pulp and solution being brought together in such amounts and concentrations as to give a resulting suspension having an alkalinity equivalent to that of a caustic soda solution of at least about 5% strength but below mercerizing strength, continuing the treatment for the desired riod of time, separating caustic, alkali solution from the so-tneated pulp ter, subjecting the treated and washed pul t9 a second stage of bleaching treatment, an

washing the resulting-finally bleached pulp. 13, In purifying unbleached chemical wood pulp, the sequence of operations which com-- prises partially bleaching the pulp, treating the partially bleached pulp with a solution of caustic alkali, the pulp and solution being trations as to give a resultingsuspension having an alkalinity equivalent to tha-tjof a cans-x tic soda solutionof at least about 5%strength but below mercerizing strength, continuing 180 F.,-with al ahne solution of suficient" the treatment for the desired period of time at a temperature less than the boiling point of water, separating caustic alkali solution from the so-treated pulp and washing the latter, subjecting the treatedand washed pulp in aqueous suspension to asecond stage of bleaching treatment, thereafter treating the pulpwith an antilchlor, and washing the resulting finally bleached pulp.

14. In purifyinghmbleached chemical wood pulp, the sequence of operations which. comprises partially bleaching the pulp, treating thepartially bleached and washed pul with a solution ofcaustic alkali, the pulp an caus- .tic alkali solution being brought together in such amounts and concentrations asto give a resultingsuspension'having an alkalinity memorizing strength, continuing the treatand washing the l-atbrought together in such amounts and concen ment for the desired period of time at a temperature below 212 F., separating caustic alkali solution from the so-treated pulp and washing the latter, subjecting the treated and washed pulp in aqueous suspension to a second stage of bleaching treatment, and washing the resulting finally bleached pu1p.'

15. In purifying unbleached chemical wood pulp, the sequence of operations which com- 10 prises partially bleaching the pulp, thickening the same, treating the thickened pulp with a solution of caustic alkali, the pulpand caustic alkali solution being brought together in such amounts and concentrations as to give a resulting suspension having an alkalinity equivalent to that of caustic soda solution of at least about 5% strength but below mercerizing strength, continuing the treatment at a temperature below 212 F. for the desired period of time, separating caustic alkali solution from the so-treated pulp and washing the 1 latter, subjecting the treated and washed pulp to a second stage of bleaching treatment, and

washing the resulting finally bleached pulp. In testimony whereof I have afiixcd my signature.

GEORGE A. RICHTER. 

